Ultrasonic Press Using Servo Motor With Delayed Motion

ABSTRACT

An ultrasonic welding system includes a movable ultrasonic welding stack, an electrically powered linear actuator with a servo motor and a movable element, a controller, and at least two sensors. The controller causes the ultrasonic welding stack to apply a predetermined positive initial force to at least one workpiece prior to initiation of welding. The controller further causes the ultrasonic welding stack to initiate subsequent movement of the ultrasonic welding stack, following initiation of welding, only after the signal outputs from the at least two sensors indicate that a combination of control variables satisfies a predetermined condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/471,895, filed Aug. 28, 2014, which is a continuation-in-part of U.S.patent application Ser. No. 14/209,273, filed Mar. 13, 2014, which is acontinuation of U.S. patent application Ser. No. 13/245,021, filed Sep.26, 2011, now issued as U.S. Pat. No. 8,720,516, which is a continuationof U.S. patent application Ser. No. 12/418,093, filed Apr. 3, 2009, nowissued as U.S. Pat. No. 8,052,816, which claims the benefit of andpriority from U.S. Provisional Patent Application No. 61/042,574, filedApr. 4, 2008, and is a continuation-in-part of U.S. patent applicationSer. No. 11/800,562, filed May 7, 2007, now issued as U.S. Pat. No.7,819,158, which claims the benefit of and priority from U.S.Provisional Patent Application No. 60/798,641, filed May 8, 2006, eachof which is incorporated herein in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to presses for use in ultrasonicwelding or other systems for vibratory joining of plastic parts.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present concepts, an ultrasonicwelding system includes a movable ultrasonic welding stack to move andto apply vibrational energy to at least one workpiece responsive tocontrol inputs. An electrically powered linear actuator, which includesa servo motor and a movable element, is coupled to the ultrasonicwelding stack and causes, responsive to control inputs, the movableelement and the ultrasonic welding stack to move with a controlledforce, speed, or force and speed. A controller provides control inputsto the electrically powered linear actuator to control an output of theelectrically powered linear actuator. At least two sensors sense controlvariables and output signal outputs corresponding to the controlvariables to the controller. The controller causes the ultrasonicwelding stack to apply a predetermined positive initial force to the atleast one workpiece prior to initiation of welding. The controllerfurther initiates subsequent movement of the ultrasonic welding stack,following initiation of welding, only after the signal outputs from theat least two sensors indicate that a combination of control variablessatisfies a predetermined condition.

In another aspect of the present concepts, a method for a weldingoperation includes initiating a welding operation by moving, in responseto control inputs, an ultrasonic welding stack to apply vibrationalenergy to a workpiece. The method further includes causing, responsiveto control inputs, a movable element of an electrically powered linearactuator to move with a controlled force, speed, or force and speed. Themethod further includes providing control inputs, via a controller, tothe electrically powered linear actuator for controlling an output ofthe electrically powered linear actuator. Control variables are sensed,via at least two sensors, and signal outputs corresponding to thecontrol variables are outputted to the controller. The controller causesthe ultrasonic welding stack to apply a predetermined initial force tothe at least one workpiece prior to initiation of welding. Subsequentmovement of the ultrasonic welding stack is initiated, followinginitiation of welding, only after the signal outputs from the at leasttwo sensors indicate that a combination of control variables satisfies apredetermined condition.

Additional aspects of the invention will be apparent to those ofordinary skill in the art in view of the detailed description of variousembodiments, which is made with reference to the drawings, a briefdescription of which is provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following descriptionof preferred embodiments together with reference to the accompanyingdrawings, in which:

FIG. 1 is a front perspective view of an ultrasonic welding machine;

FIG. 2 is an enlarged side perspective of a portion of the ultrasonicwelding machine shown in FIG. 1, with portions of the housing wallsbroken away to reveal the internal structure, including the linearactuator.

FIG. 3 is a variation of FIG. 2 showing a linear motor drive in place ofthe servo-motor driven actuator.

FIG. 4 is a variation of FIG. 2 showing a load cell used for forcefeedback.

FIG. 5 is an enlarged, exploded elevation of the ultrasonic “stack” inthe ultrasonic welding machine shown in FIG. 1.

FIG. 6 is a variation of FIG. 5 showing a spring-loaded contact buttonwhich remains pressed against a contact bar.

FIG. 7 is a block diagram of one embodiment of a control system for thelinear actuator used in the ultrasonic welding machine shown in FIGS.1-4.

FIG. 8 is a block diagram of one embodiment of a control system for thelinear actuator used in the ultrasonic welding machine shown in FIG. 4.

FIG. 9 shows a distance versus time graph for a weld sample formed usinga servo press and employing a delayed motion technique in accord with atleast one aspect of the present concepts.

FIG. 10 shows a force versus time graph for the weld in the sample notedin FIG. 9.

FIG. 11 shows a power versus time graph for the power output to thetransducer of the weld stack for the weld in the sample noted in FIG. 9.

FIG. 12 shows an example of corresponding distance, force, and forcerate of change graphs plotted versus time for a delayed weld motion.

FIG. 13 shows another example of corresponding distance, force, andultrasound amplitude graphs plotted versus time for a delayed weldmotion.

FIG. 14 shows another example of corresponding distance and force graphsplotted versus time for a weld motion with two motion delay phases.

FIG. 15 shows another example of corresponding speed, force, and forcerate of change graphs plotted versus time for a weld (a) without and (b)with dynamic speed adjustment.

FIG. 16 shows another example of corresponding speed and force graphsplotted versus time for a weld (a) without and (b) with dynamic speedadjustment.

FIG. 17 shows another example of corresponding ultrasound amplitude andspeed graphs plotted versus time for a weld in which speed is ramped upin direct proportion to ultrasound amplitude.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Although the invention will be described in connection with certainpreferred embodiments, it will be understood that the invention is notlimited to those particular embodiments. On the contrary, the inventionis intended to cover all alternatives, modifications, and equivalentarrangements as may be included within the spirit and scope of theinvention as defined by the appended claims.

Turning now to the drawings and referring first to FIGS. 1-6, theillustrative ultrasonic welding machine includes an ultrasonic welding“stack” 10 that is mounted for controlled vertical movement by abidirectional, electrically powered linear actuator 11 (FIG. 2). Thestack 10 will be described in more detail below in connection with FIGS.5 and 6. The actuator 11 is mounted within a main housing 12, which alsosupports an auxiliary housing 13 that contains the power supply andelectronic controls for the welding press. In a variation of thisconcept, the housing 12 and auxiliary housing 13 may be combined intoone structure without materially affecting the intent of this invention.The thermoplastic workpieces W1 and W2 (FIG. 5) to be welded are mountedin a stationary fixture below the ultrasonic stack 10, and the actuator11 advances the stack 10 downwardly against the upper workpiece W1. Thelower end of the stack 10 is pressed downwardly against the workpiece W1to press the upper workpiece W1 against the lower workpiece W2 whileapplying mechanical vibrations to the workpiece W1 to effect the desiredwelding that joins the two workpieces W1 and W2 together.

The main housing 12 is mounted on a frame that includes a verticalcolumn 14 extending upwardly from a base 15 that carries a fixture forreceiving and supporting the workpieces to be welded. The housing 12 istypically adjustably mounted on the column 14 to allow the verticalposition of the entire housing 12 to be adjusted for differentworkpieces. A control panel 16 is provided on the front of the base 15.

The ultrasonic welding stack 10 includes the following three components(see FIGS. 5 and 6):

-   -   an electromechanical transducer 20 which converts electrical        energy into mechanical vibrations;    -   a booster 21 to alter the gain (i.e., the output amplitude) of        the mechanical vibrations produced by the transducer 20; and    -   a horn 22 to transfer the mechanical vibrations from the booster        21 to the parts to be welded.

As shown in FIG. 5, the transducer 20 includes a connector 23 forattaching a high voltage coaxial cable 24 that delivers a high-frequencyelectrical signal for exciting the transducer 20. This signal issupplied by a separate ultrasonic signal generator (not shown). Analternative method of connection can also be utilized to permit easierremoval and installation of the transducer. This method as shown in FIG.6 utilizes a spring mounted button on the transducer 20 which contacts aconductive bar on the press. Electrical conductivity is insured by thespring force behind the button as it presses against the bar.

The transducer 20 generates the ultrasonic vibrations as a Langevinpiezoelectric converter that transforms electrical energy intomechanical movement. Power applied to the transducer 20 can range fromless than 50 Watts up to 5000 Watts at a typical frequency of 20 kHz.Note that the same concepts will hold true for transducers of otherfrequencies and power levels which are regularly used in the weldingprocesses of this invention.

The transducer 20 is typically made from a number of standardpiezoelectric ceramic elements separated by thin metal plates, clampedtogether under high pressure. When an alternating voltage is applied tothe ceramic elements, a corresponding electric field is produced whichresults in a variation in thickness of the ceramic elements. Thisvariation in thickness induces a pressure wave that propagates throughthe material and is reflected by the ends of the metal mass of thetransducer. When the length of the assembly is tuned to its frequency ofexcitation, the assembly resonates and becomes a source of standingwaves. The output amplitude from a 20-kHz transducer is typically about20 microns (0.0008 inches). This amplitude needs to be amplified by thebooster 21 and the horn 22 to do useful work on the parts W1 and W2. Thebooster and horn act as an acoustic waveguide or transformer to amplifyand focus the ultrasonic vibrations to the work piece.

The primary function of the booster 21 is to alter the gain (i.e.,output amplitude) of the stack 10. A booster is amplifying if its gainis greater than one and reducing if its gain is less than one. Gains at20-kHz typically range from less than one-half to about three.

The horn 22 cannot normally be clamped because it must be free tovibrate and thus only the transducer 20 and the booster 21 are secured.Thus, a secondary function (and sometimes the sole purpose) of thebooster is to provide an additional mounting location without alteringthe amplification of the stack when secured in a press. The neutral orcoupling booster is added between the transducer and horn and mounted inthe press by a mounting ring which is placed at the nodal point (wherethe standing wave has minimal longitudinal amplitude).

The horn 22 has three primary functions, namely:

-   -   it transfers the ultrasonic mechanical vibrational energy        (originating at the transducer 20) to the thermoplastic work        piece (W1 and W2) through direct physical contact, and localizes        the energy in the area where the melt is to occur;    -   it amplifies the vibrational amplitude to provide the desired        tip amplitude for the thermoplastic workpiece and welding        process requirements; and    -   it applies the pressure necessary to force the weld when the        joint surfaces are melted.

The horn is precision machined and is typically designed to vibrate ateither 15 kHz, 20 kHz, 30 kHz, 40 kHz, 50 kHz or 70 kHz. The higher thefrequency, the shorter the acoustic wavelength, and consequently thesmaller the horn. The tuning of a horn is typically accomplished usingelectronic frequency measurement. Horns are usually manufactured fromhigh-strength aluminum alloys or titanium, both of which have excellentacoustical properties to transmit the ultrasonic energy with littleattenuation.

There are many different horn shapes and styles depending on the processrequirements. Factors which influence the horn design are the materialsto be welded and the method of assembly. The horn must amplify themechanical vibration so that the amplitude is sufficient to melt thethermoplastic workpieces at their interface, and the gain of the horn isdetermined by its profile. The amplitude at the tip of the horntypically ranges from 30 to 125 microns peak to peak (1.2 to 5.0thousandths of an inch) at 20 kHz. In an alternate variation, the horncan be designed so that it takes the form of a booster and combines thefunctions of stabilization and welding. In this variation, the boosteris eliminated and the horn is secured in the press in the position ofthe booster mounting ring area.

As the frequency increases, the vibration amplitude decreases. Higherfrequencies are used for seaming of thin materials and delicate partsthat do not require a lot of amplitude. Since the horn becomes smallerat higher frequencies, closer spacing can also be achieved.

Plastic welding is the most common application of ultrasonic assembly.To perform ultrasonic plastic welding, the tip of the horn is broughtinto contact with the upper workpiece W1, as shown in FIG. 5. Pressureis applied and ultrasonic energy travels through the upper workpiece,increasing the kinetic energy (or heat) at the contact point of the twoworkpieces. The heat melts a molded ridge of plastic on one of theworkpieces, and the molten material flows between the two surfaces. Whenthe vibration stops, the material solidifies forming a permanent bond.

The linear actuator 11 comprises an electric servo motor 30 integratedwith a converter 31 that converts the rotating output of the motor 30into linear motion. The converter is typically a lead screw coupled tothe motor output shaft 30 a, with a follower unit traveling along thethreads of the lead screw to produce the desired linear output. In theillustrative embodiment, the linear output is controlled verticalmovement of a rod 31 a that connects the converter 31 to the stack 10.The integrated unit that contains both the servo motor 30 and theconverter 31 is a commercially available item, such as the GSM or GSXSeries linear actuators available from Exlar Corporation of Chanhassen,Minn. See also U.S. Pat. No. 5,557,154 assigned to Exlar Corporation.The linear position feedback used by the servo motor can be provided bya linear encoder coupled to the weld stack 10, or by a rotary encoderwhich senses the position of the rotating motor 30.

As can be seen in FIGS. 2 and 4, the actuator rod 31 a moves linearlyalong a vertical axis. The lower end of the rod 31 a is connected to thecomponents comprising the carriage to which the ultrasonic welding stack10 is attached. The purpose of the actuator 11 is to apply a controlledforce, speed, or a combination of force and speed to the stack 10 topress the stack downwardly against the workpiece W1 while the stack isalso transmitting mechanical vibrations to the workpiece. The linearmovement of the rod 31 a is another controllable variable. For example,the linear movement of the rod 3 a may be controlled so as to control aweld depth, especially after the thermoplastic material of theworkpieces has been softened sufficiently to effect the desired weld.Excessive advancement of the rod 3 a after the thermoplastic materialhas been softened by the applied vibrating energy can produce a weldthat is too thin and, therefore, too weak. Likewise, in accord withconcepts disclosed below, an initial linear movement of the rod 31 a maybe delayed, such as by being held at or near zero, until after asoftening of the thermoplastic material of the workpieces causes areduction in an initially applied force to a level below a predeterminedthreshold.

An alternative method of driving the welding stack is shown in FIG. 3 bythe use of a direct drive linear servo slide. These slides reduceinaccuracies caused by gear backlash and power screw wrap up. A directdrive linear servo motor 38 acts on the stack assembly 10. This lineardrive servo motor is a combination of the motor 30 and the converter 31.Such drives are commercially available from a number of suppliers suchas the Parker Trilogy 410 Series. The position feedback 36 is provideddirectly by the linear motor, e.g., using an encoder or resolver coupleddirectly to the motor shaft. In order to use a linear servomotor in avertical configuration, a separate, electric brake 37 is required tokeep the welding stack 10 from falling under its own weight during poweroff conditions.

FIG. 7 illustrates a control system for the linear actuator 11. A forcecontrol loop includes a torque sensor 32 coupled to the rotary outputshaft 30 a of the electrical servo motor 30, for producing an electricalsignal related to the magnitude of the torque output of the motor 30.This torque signal is processed in conventional signal conditioningcircuitry 33 and then supplied to a motion controller 34 that receivespower from a power source 35 and controls the electrical currentsupplied to the motor 30 via drive amplifier 34 a. Thus, the torquesensor 32 and the signal conditioning circuitry 33 form a feedback loopthat controls the motor 30 to turn the output shaft 30 a with a desiredtorque, which in turn controls the force applied to the stack 10 by theconverter 31 that converts the rotary output of the motor 30 to linearmotion of the rod 31 a. This feedback loop makes it possible to controlthe pressure applied to the workpieces during the welding operation bycontrolling the output torque produced by the servo motor.

An alternate method of providing force feedback to the control systemuses a commercially available load cell in place of torque control onthe motor drive itself. The load cell 40 is positioned so that it canmeasure the force exerted by the welding stack upon the work piece. Thisis illustrated in FIGS. 4 and 8.

To control the magnitude of the linear displacement of the rod 31 a, aposition sensor 36 is coupled to the rod 31 a, for producing anelectrical signal related to the vertical movement of the rod 31 a. Forexample, the position sensor 36 may be an encoder that produces a numberof electrical pulses proportional to the magnitude of the displacementof the rod 31 a. This position signal is supplied to the controller 34as a further parameter for use by the controller 34 in controlling theelectrical current supplied to the motor 30. Thus, the position sensor36 is part of a feedback loop that controls the motor 30 to control theangular displacement of the output shaft 30 a, which in turn controlsthe magnitude of the vertical movement of the rod 31 a, and thus of thestack 10. The actual displacement of the stack 10 is, of course, afunction of both the force applied by the motor 30 and the resistanceoffered by the workpieces, which varies as the weld zone is heated andsoftens the thermoplastic material of the workpieces.

An alternate method of determining the linear position of the weldingstack during the welding cycle is by utilizing the encoder feedback ofthe motor. This is represented by item 41 in FIG. 7 or item 36 in FIG.8. This position is a function of motor position and the drive screw nutlead in combination with any gear reduction used in the drivetrain.

In addition to controlling the force, speed, or combination of force andspeed directly, the motion control system 34 is capable of automaticallychanging the force or speed on-the-fly based on an arbitrary algorithmusing an input signal or combination of signals from an external controldevice 42. The external control device 42 may be the ultrasonicgenerator or controller which provides power and control to the stack10. It may be a controller which is connected to or involved with theworkpieces W1 and W2. In these instances the motion controller 34receives the input signal(s) from an external device 42, signalconditioner 33, and position sensor 36 and generates the force or speedchanges during the welding and holding processes. For example, theactuator can be commanded to automatically change force or speed in aneffort to maintain ultrasound power output (provided by ultrasonicgenerator) constant. As a second example, the ultrasonic transducer 20may provide feedback power to an external control device 42 related tothe force being exerted upon it. This feedback power will be used as abasis for the external control device to influence the motion controller34 to change the force or speed of the motor and actuator rod 30 and 31a. The result will be a closed servo-control loop relating the forceapplied to the workpiece W1 and W2 and the actual welding speed asreported by either or both of the position sensors 36 and 41.

There are numerous advantages of using servo-electric control in awelding system of this type. The primary advantage is the capability toprecisely control the position of the welding stack throughout the weldprocess due to the repeatable and controllable nature of electricalpower compared with pneumatic systems, which are subject to inaccuraciesdue to media compressibility. A second advantage is ability to changethe speed or force of the weld stack faster from one level to anotherusing a servo system. A third advantage is the increased ease ofcalibration and verification of a welding system using an electric servodue to absence of all pneumatic controls, which also reduces the effortinvolved in setting up multiple welding systems to achieve matchingperformance.

It is also possible to combine the effects of the speed and forcefeedback to control the weld process. An example of this is monitoringand varying the speed as a secondary control in order to hold a constantforce exerted by the servo motor on the part. In this scenario a maximumand minimum welding speed can be defined to ensure that all parts have awell defined envelope of process parameters. The reciprocal method ofvarying the force exerted by the servo motor within defined limits tomaintain a predetermined velocity profile is also viable with thisapparatus and the control capabilities inherent in the design. As oneexample, the ultrasonic welding method includes at least one inputsignal to adjust the force or speed of the linear actuator responsive toa measured power (e.g., an instantaneous power) delivered to thetransducer 20. In another example, the ultrasonic welding methodincludes at least one input signal to adjust the force or speed of thelinear actuator responsive to a cumulative power delivered to thetransducer 20 (i.e., the power delivered to the transducer iscontinually summed over time to yield the cumulative power, and thiscumulative power may be used as the reference in a feedback loop).

FIG. 9 shows a distance versus time graph for a polycarbonate weldsample formed using a servo press system and employing a delayed motiontechnique in accord with at least one aspect of the present concepts.FIG. 10 shows a force versus time graph for the weld in the sample notedin FIG. 9. FIG. 11 shows a power versus time graph for the power outputto the transducer of the weld stack for the weld in the sample noted inFIG. 9. In this depicted experimental weld sample, a feature wasimplemented wherein, after an initial load (“trigger force”) of 20pounds was applied to the ultrasonic stack, the displacement of theultrasonic weld stack 10 was held substantially at zero. It bears notingthat the initial load is a variable load that is selectable by anoperator or, alternatively by the control system upon input ofappropriate welding parameters and process information, and may varybetween zero pounds and any upper limit of the linear actuator utilized.After this initial load was applied, the welding operation was initiatedat a time of 0 seconds by powering the transducer 20 of the ultrasonicwelding stack 10. At that time, the weld collapse distance was 0 inches.Through the time of about 0.080 seconds, the weld distance wasmaintained substantially at 0 inches.

During this time, the ultrasonic weld stack 10 power increased and thewelding operation began to soften the thermoplastic material of theworkpiece at the welding point. Correspondingly, a drop in force (FIG.10) starting at a time of about 0.064 seconds is observed. At this time,the power to the transducer 20 is about 275 W (see FIG. 11). Betweenabout 0.064 seconds and about 0.080 seconds, the force applied by thelinear actuator 11 on the ultrasonic weld stack 10 is observed to dropfrom about 26 pounds to about 9 pounds. Up until this time, the welddistance is maintained near zero and the linear actuator rod 3 a andultrasonic weld stack 10 are not appreciably advanced. However,following the observed decrease in force past a selected predeterminedthreshold force, which was about 17 pounds in the present example, thecontrol system initiated downward motion of the weld stack (e.g., apositive downward velocity) to continue the weld process in accord witha selected weld process profile, as indicated by the parameters in FIGS.9-11.

The weld sample produced by the weld process depicted in FIGS. 9-11 wasmeasured, yielding a collapse height (e.g. difference between unweldedand welded parts) of 0.0174 inches, and subsequently pull tested,yielding an ultimate pull strength of 1006 pounds. In testing of theconcepts described herein, a statistically significant number of sampleswere welded under similar conditions (i.e., implementing a delayedmotion technique as described herein) and yielded an average collapseheight of 0.0172 inches with a standard deviation of 0.0001 inches, andpull strength of 991 pounds with a standard deviation of 19 pounds.Comparison tests were performed on another group of the same weldsamples using a pneumatic system with the same ultrasonic weld horn andgenerator. In the pneumatic tests, the ultrasonic weld stack wasoperated in a “force” mode wherein a specified weld force is maintainedby controlling air pressure to achieve a fairly constant weld forcethroughout the weld. By comparison, a statistically significant numberof samples produced by the pneumatic system weld process were measured,yielding an average collapse height of 0.0182 inches with a standarddeviation of 0.0005 inches, and pull tested, yielding an average pullstrength of 1002 pounds, with a standard deviation of about 31 pounds.

The results of the servo tests implementing the delayed motion techniquewere superior to those of the pneumatic tests for consistency ofcollapse distance and pull strength repeatability. In addition, althoughthe absolute average value of the pull strength was slightly higher withthe pneumatic system, the average weld collapse distance was alsoslightly higher. Since these samples employed a shear weld joint designfamiliar to those skilled in the art, the average pull strengths perunit of weld collapse distance can be compared. The samples welded onthe servo system yielded a higher relative strength compared to thesamples welded on the pneumatic system. The average values were 57,700and 55,200 pounds per inch of weld collapse, respectively.

It is expected that still further improvements to weld strength may beobtained by adjusting the amount of the delay before initiating adownward motion of the ultrasonic welding stack 10 as well as byadjusting the velocity profile throughout the remainder of the weld.Improvements to strength repeatability can also be expected by enhancingthe accuracy and repeatability of force sensing employed in thistechnique, which can be achieved by further reducing electrical andmechanical noise in the sensing circuitry.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.As one example, although the weld distance of the ultrasonic weldingstack has been described herein in the delayed motion phase of thewelding operation to be maintained at or near zero, a slight slope or anarbitrary profile may be advantageously used.

As another example, in accord with at least some aspects of the presentconcepts, it is possible that the described actuator and associatedcontrol system could be implemented in combination with the secondworkpiece W2 such that the actuator moves the second workpiece W2 towardthe stationary workpiece W1 attached to or adjacent a stationary weldingstack (i.e., stationary except for the vibratory movement of the horn22). The control systems described herein then control a linear movementof the second workpiece W2 against the first workpiece W1 by applying acontrolled force, speed, or a combination of force and speed to thesecond workpiece with the electrically powered linear actuator to urgethe second workpiece against the first workpiece to which the secondworkpiece is to be joined. Likewise, another potential application ofthe present concepts may include an arrangement wherein the secondworkpiece W2 is mounted adjacent the horn of the ultrasonic weldingstack and the described actuator and associated control systemimplemented as previously described to bias the first workpiece W1against the stationary workpiece W2 attached to or adjacent thestationary welding stack (i.e., stationary except for the vibratorymovement of the horn 22). The control systems described herein thencontrol a linear movement of the first workpiece W1 against the secondworkpiece W2. It is further to be understood that although forces may beshown to be applied in a particular manner herein, such as pressingagainst a stationary target workpiece from above, other variants offorce application are included within the present concepts, such as, butnot limited to, pulling a movable workpiece (e.g., W1) toward astationary workpiece (e.g., W2) in like manner.

Referring to FIG. 12, an example illustrates new conditions forinitiating a weld motion based on parameter rates of change. Asdescribed in more detail above, a predetermined condition for initiatinga weld motion includes crossing a prescribed threshold of a sensedparameter, such as a force parameter. Additionally, a weld motion isinitiated when the predetermined condition is based on crossing aprescribed threshold of the rate of change of the sensed parameter.

For example, the weld motion is delayed until the time rate of change offorce crosses a specified or predetermined level. In accordance with thespecific example of FIG. 12, “0” (zero) on the time axis corresponds tothe initiation of ultrasonic vibrations, and the condition forinitiating the weld motion is the force rate of change falling to aprescribed level. The graph of distance plotted versus time indicatesthat a weld press remains stationary for an initial phase of the weld.During this initial phase, a press force initially rises from an initialforce F_(i), reaches an apex, and then falls as a plastic material ofthe parts being welded softens and begins to melt. A corresponding graphof the force rate of change, which is the slope of the force curve,shows that the force rate of change is initially positive, thendecreases to “0” (zero), and subsequently becomes negative. When theforce rate of change reaches a predetermined level −F_(d)′, the weldmotion is initiated. In the example of FIG. 12, the weld motion isdepicted by a linear increase in the distance beyond a time t₁.

According to other examples, rates of change of other parameters may beused as a basis for the predetermined condition. For example, the ratesof change may include a power input rate of change for a transducer, afrequency rate of change for an ultrasonic stack, and/or a phase rate ofchange for a transducer.

According to an exemplary embodiment A1, a sensor senses the rate ofchange of a power input to a transducer of an ultrasonic welding stackand a predetermined condition is one or more of a specified rate ofchange of power or a specified rate of change of cumulative power.

According to another exemplary embodiment A2, a sensor senses a rate ofchange of a frequency of an ultrasonic welding stack and a predeterminedcondition is a specified rate of change of frequency.

According to yet another exemplary embodiment A3, a sensor senses a rateof change of a phase of a transducer of an ultrasonic welding stack anda predetermined condition is a specified rate of change of phase.

According to yet another exemplary embodiment A4, a sensor senses a rateof change of a force output by a linear actuator movable element and apredetermined condition is a specified rate of change of force.

According to yet another exemplary embodiment A5, a sensor senses a rateof change of an output torque of a servo motor and a predeterminedcondition is a specified rate of change of output torque.

Referring to FIG. 13, an illustrative enhancement of conditions forinitiating a weld motion is described in more detail. Theabove-described criteria for initiating a weld motion are sufficient topositively affect the weld process in most cases. However, under certainconditions, including when a welding system has not been programmed inan optimal manner based on lack of operator experience, benefits ofinitiating the weld motion may not be fully realized. According to onespecific example, if the predetermined condition for initiating the weldmotion is a specified force X1 and the difference dX between thespecified force X1 and a predetermined positive initial force X0 isprogrammed to be too small, the specified force X1 may be reached beforesufficient time elapses for the amplitude of the ultrasonic stack toramp up to a sufficiently high level to cause melting. For example, onereason for reaching the specified force X1 too soon is the fluctuationin force caused by subtle changes in alignment of the workpieces whenultrasonic vibrations are initiated.

To increase the likelihood of fully realizing the benefits of delayingthe weld motion, criteria for delaying the weld motion can be enhancedby combining multiple conditions. For example, instead of relying on asingle condition, the weld motion is initiated by utilizing multipleconditions based on input from multiple sensors. By way of example, theweld motion is delayed until both of the following conditions aresatisfied: (a) the force decreases below a specified threshold, and (b)the ultrasound amplitude increases above a specified threshold. In thisexample, a premature initiation of the weld motion, which may occur witha strictly force-based criterion, is prevented by ensuring that anappreciable amount of energy is being transferred to the parts tocommence melting.

Such an example is illustrated in FIG. 13, which shows graphs of welddistance plotted versus time, force plotted versus time, and ultrasoundamplitude plotted versus time. “0” (zero) on the time axis correspondsto the initiation of the ultrasonic vibrations. The press is stationaryduring the initial phase of the weld, as indicated by the graph ofdistance versus time. During this phase, the press force rises from theinitial force F_(i), reaches an apex, then falls as the plastic materialof the parts being welded softens and beings to melt. Concurrently, theultrasound amplitude increases with time at a predetermined rate from“0” (zero) to a programmed weld amplitude A_(w). Although the forcedecreases below a prescribed level F_(d) at t₁, the weld motion isdelayed until the amplitude increases above a prescribed level A_(d),which occurs at time t₂. The programmed weld motion follows, which inthis case is depicted by a linear increase in distance beyond time t₂.According to other examples, other combinations of conditions can beused to achieve similar benefits.

According to an exemplary embodiment B1, an ultrasonic welding systemincludes a movable ultrasonic welding stack to move and to applyvibrational energy to at least one workpiece responsive to controlinputs. The system also includes a motion control system to control amotion of the ultrasonic welding stack. The system further includes acontroller to provide control inputs to the motion control systemassociated with the ultrasonic welding stack to control movement of theultrasonic welding stack and to control an output of the ultrasonicwelding stack to the at least one workpiece. The system further includesat least two sensors to sense control variables and to output signalscorresponding to the control variables to the controller. The controllercauses the ultrasonic welding stack to apply a predetermined positiveinitial force to the at least one workpiece prior to initiation ofwelding and to initiate subsequent movement of the ultrasonic weldingstack, following initiation of welding, only after the signal outputsfrom the at least two sensors indicate that a combination of controlvariables satisfies a predetermined condition.

According to another exemplary embodiment B2, the sensors of embodimentB1 sense a force output by the linear actuator movable element andamplitude of the ultrasonic stack. The predetermined condition includesboth a specified force and a specified amplitude.

According to yet another exemplary embodiment B3, the sensors ofembodiment B1 sense a force output by the linear actuator movableelement and a power input to a transducer of the ultrasonic weldingstack. The predetermined condition includes both a specified force and aspecified power.

According to yet another exemplary embodiment B4, the sensors ofembodiment B1 sense a force output by the linear actuator movableelement and a power input to a transducer of the ultrasonic weldingstack. The predetermined condition includes both a specified force and aspecified cumulative power.

According to yet another exemplary embodiment B5, one sensor ofembodiment B1 senses a force output by the linear actuator movableelement and another sensor of embodiment B1 tracks elapsed timefollowing the initiation of welding. The predetermined conditionincludes both a specified force and a specified elapsed time.

According to yet another exemplary embodiment B6, the sensors ofembodiment B1 sense an output torque of the servo motor and an amplitudeof the ultrasonic welding stack. The predetermined condition includesboth a specified output torque and a specified amplitude.

According to yet another exemplary embodiment B7, the sensors ofembodiment B1 sense an output torque of the servo motor and a powerinput to a transducer of the ultrasonic welding stack. The predeterminedcondition includes both a specified output torque and a specified power.

According to yet another exemplary embodiment B8, the sensors ofembodiment B1 sense an output torque of the servo motor and a powerinput to a transducer of the ultrasonic welding stack. The predeterminedcondition includes both a specified output torque and a specifiedcumulative power.

According to yet another exemplary embodiment B9, one sensor ofembodiment B1 senses an output torque of the servo motor and anothersensor tracks elapsed time following the initiation of welding. Thepredetermined condition includes both a specified output torque and aspecified elapsed time.

According to yet another exemplary embodiment B10, the sensors ofembodiment B1 sense one or more of a force output by the linear actuatormovable element, an output torque of the servo motor, an amplitude ofthe ultrasonic welding stack, a power input to a transducer of theultrasonic welding stack, a cumulative power input to a transducer ofthe ultrasonic welding stack, a frequency of the ultrasonic weldingstack, and a phase of a transducer of the ultrasonic welding stack,and/or one or more of the sensors track an elapsed time following theinitiation of welding. The predetermined condition includessimultaneously satisfying two criteria, including a first criterion anda second criterion, each criterion being associated with distinct sensedcontrol variables. The first criterion including one of a specifiedforce, output torque, amplitude, power, cumulative power, frequency,phase, or elapsed time. The second criterion including one of aspecified force, output torque, amplitude, power, cumulative power,frequency, phase, or elapsed time.

Referring to FIG. 14, an alternative embodiment includes delaying theweld motion after some weld motion has already occurred. As describedabove, an ultrasonic welding system that employs a delayed motionapplies to the initial stage of the weld phase, where the plastic partsbeing joined are starting to melt. However, for some applications thistechnique can also be beneficial when utilized during one or more latterphases of the weld. For example, on plastic parts where, by design,there is more volume of material to melt in one area relative to otherareas, the rate of melt may not be uniform, resulting in unequallywelded sections. In such cases, overall weld uniformity is improved bytemporarily suspending the weld motion (e.g., maintaining the positionof the weld stack) until the force decreases by a prescribed amount andadditional melting occurs in the areas of concentrated material.

By way of a specific example, illustrated in FIG. 14, graphs of welddistance and force plotted versus time are shown for a weld with twomotion delay phases. “0” (zero) on the time axis corresponds to theinitiation of ultrasonic vibrations. The first delay that occurs in thetime interval between time “0” and t₁ is generally similar to the welddelays described above. The second delay occurs after some weld motionhas already taken place. In this embodiment, the second delay isinitiated when the force reaches a prescribed level F_(d2i), which isrepresented at time t₂, and the second delay is terminated when theforce decreases by a prescribed amount ΔF_(d2), to F_(d2t), which isrepresented at time _(t3).

The second motion delay can be initiated by one or more of a number ofparameters, including, for example, a force threshold, a powerthreshold, a cumulative power threshold, and/or a distance traversedfrom the start of the weld. The second motion delay can be terminated byone or more of a number of parameters, including, for example, a changein the force or power from the level sensed at the time of theinitiating condition, and/or the amount of time that elapses from themoment of the initiating condition. The number of times that the weldmotion may be delayed in a single weld cycle can vary from a singledelay to numerous delays, as needed, based on specific weldrequirements. Additionally, the delay at the initiation of ultrasonicvibrations may be omitted while one or multiple delays are subsequentlyemployed after some weld motion has already occurred.

According to an exemplary embodiment C1, an ultrasonic welding systemincludes an ultrasonic welding stack mounted for linear movement andapplying a controller force, speed, or a combination of force and speedto a first workpiece to urge the first workpiece against a secondworkpiece to which the first workpiece is to be joined. The system alsoincludes an electrically powered linear actuator including a movableelement coupled to the ultrasonic welding stack, the electricallypowered linear actuator causing, responsive to control inputs, themovable element and the ultrasonic welding stack to move with acontrolled force, speed, or force and speed. The system further includesa controller to provide control inputs to at least one of theelectrically powered linear actuator or the servo motor to control anoutput of the electrically powered linear actuator. The system furtherincludes one or more sensors to measure at least one correspondingcontrol variable and to output a signal corresponding to the controlvariable to the controller. The controller, based on the signal outputby at least one sensor, causes the electrically powered linear actuatormovable element to stop motion and maintain a stationary position,subsequent to any preceding weld motion, from a predetermined delayinitiating condition until a predetermined delay terminating condition.The controller, based on the signal output by at least one sensorindicating that the predetermined delay terminating condition has beensatisfied, further causes the electrically powered linear actuator toresume motion of the ultrasonic welding stack in accordance with adefault weld profile or a weld profile selected from a plurality ofavailable weld profiles.

According to another exemplary embodiment C2, at least one sensor ofembodiment C1 senses a force output by the linear actuator movableelement. The predetermined delay initiating condition is a specifiedforce and the predetermined delay terminating condition is a specifiedchange in force from the level sensed at the time of initiatingcondition.

According to yet another exemplary embodiment C3, at least one sensor ofembodiment C1 senses a distance traversed from the start of the weld anda force output by the linear actuator movable element. The predetermineddelay initiating condition is a specified distance and the predetermineddelay terminating condition is a specified change in force from thelevel sensed at the time of the initiating condition.

According to yet another exemplary embodiment C4, at least one sensor ofembodiment C1 senses a power input to a transducer of the ultrasonicwelding stack and a force output by the linear movable element. Thepredetermined delay initiating condition is a specified power and thepredetermined delay terminating condition is a specified change in forcefrom the level sensed at the time of the initiating condition.

According to yet another exemplary embodiment C5, at least one sensor ofembodiment C1 senses a power input to a transducer of the ultrasonicwelding stack. The predetermined delay initiating condition is aspecified power and the predetermined delay terminating condition is aspecified change in power from the level sensed at the time of theinitiating condition.

According to yet another exemplary embodiment C6, at least one sensor ofembodiment C1 senses a power input to a transducer of the ultrasonicwelding stack and a force output by the linear movable element. Thepredetermined delay initiating condition is a specified cumulative powerand the predetermined delay terminating condition is a specified changein force from the level sensed at the time of the initiating condition.

According to yet another exemplary embodiment C7, one sensor ofembodiment C1 senses a force output by the linear actuator movableelement and another sensor tracks elapsed time from the moment of thedelay initiating condition. The predetermined delay initiating conditionis a specified force and the predetermined delay terminating conditionis a specified elapsed time.

According to yet another exemplary embodiment C8, one sensor ofembodiment C1 senses a distance traversed from the start of the weld andanother sensor of embodiment C1 tracks elapsed time from the moment ofthe delay initiating condition. The predetermined delay initiatingcondition is a specified distance and the predetermined delayterminating condition is a specified elapsed time.

According to yet another exemplary embodiment C9, one sensor ofembodiment C1 senses a power input to a transducer of the ultrasonicwelding stack and another sensor of embodiment C1 tracks elapsed timefrom the moment of the delay initiating condition. The predetermineddelay initiating condition is a specified power and the predetermineddelay terminating condition is a specified elapsed time.

According to yet another exemplary embodiment C10, one sensor ofembodiment C1 senses a power input to a transducer of the ultrasonicwelding stack and another sensor of embodiment C1 tracks elapsed timefrom the moment of the delay initiating condition. The predetermineddelay initiating condition is a specified cumulative power and thepredetermined delay terminating condition is a specified elapsed time.

According to yet another exemplary embodiment C11, one or more sensorsof embodiment C1 sense one or more of a force output by the linearactuator movable element, an output torque of the servo motor, aposition of the ultrasonic stack, a distance traversed from the start ofthe weld, a power input to a transducer of the ultrasonic welding stack,a cumulative power input to a transducer of the ultrasonic stack, afrequency of the ultrasonic stack, and a phase of a transducer of theultrasonic stack, and/or one or more sensors of embodiment C1 trackselapsed time following the initiation of welding. The predetermineddelay initiating condition includes one or more of a specified force, anoutput torque, a position, a distance, a power, a cumulative power, afrequency, a phase, and/or an elapsed time. The predetermined delayterminating condition includes one or more of a specified absolute orrelative value of a force, an output torque, a power, a cumulativepower, a frequency, a phase, and/or an elapsed time. The reference for arelative value is the level of the particular parameter sensed at thetime of the initiating condition.

According to yet another exemplary embodiment C12, one or more sensorsof embodiment C1 sense the present values and the rates of change of oneor more of a force output by the linear actuator movable element, anoutput torque of the servo motor, a position of the ultrasonic stack, adistance traversed form the start of the weld, a power input to atransducer of the ultrasonic welding stack, a cumulative power input toa transducer of the ultrasonic stack, a frequency of the ultrasonicstack, a phase of a transducer of the ultrasonic stack, and/or one ormore sensors of embodiment C1 track elapsed time following theinitiation of welding. The predetermined delay initiating conditionincludes one or more of a specified force, an output torque, a position,a distance, a power, a cumulative power, a frequency, a phase, and/or anelapsed time. The predetermined delay terminating condition includes oneor more of a specified rate of change of a force, an output torque, apower, a cumulative power, a frequency, and/or a phase.

Referring to FIG. 15, an alternative embodiment includes dynamic speedadjustment to reduce rapid changes in force. When controlling the speedof motion of an ultrasonic stack during the weld and hold(solidification) phases of a joining cycle, the resulting force betweenthe work pieces varies according with the changing conditions of theparts being welded. In some cases, the rate of change in the force,positive or negative, is high and can have detrimental effects on thequality of the weld. To reduce this rate of change, the speed of thepress can be dynamically adjusted by the welding system.

If the force is detected to be increasing at an excessively high rate,the press automatically reduces speed. Conversely, if the force isdecreasing at an excessively high rate, the press automaticallyincreases speed. The condition for initiating an automatic speed changeis a specified rate of change in the force. The amount of speed changecan be a predetermined fraction or multiple-factor of the current speed.For example, the speed change can include a reduction in speed of 100%relative to the current speed, or can include an increase in speed inexcess of 100% of the current speed.

Alternatively, the amount of speed change can be assigned dynamically inproportion to a detected rate of change of force. In other words, alarger speed change would be commanded for a higher rate of change inthe force. Several conditions can be used for terminating the speedchange, including, for example, crossing a prescribed level in the rateof change of force and/or a return of the rate of change of force to thelevel at which the speed change was initiated.

According to the example illustrated in FIG. 15, graphs of weld speed,force, and force rate of change plotted versus time illustrate a weld(a) without and (b) with dynamic speed adjustment. Referring to the weldwithout the dynamic speed adjustment, the speed during the weld isconstant at S_(w), which results in a continuous increase in the forcerate of change as determined by the slope of the force curve. When thedynamic speed adjustment is employed, the speed is automatically reducedto a prescribed level 0.5 S_(w) (i.e., half of S_(w)) when the forcerate of change increases to a prescribed level F1′, which is representedat time t₁. This reduction subsequently causes a decrease in the forcerate of change until it reaches a prescribed value of “0” (zero), whichis represented at time t₂. The speed is, then, automatically reverted tothe programmed weld speed S_(w).

According to an exemplary embodiment D1, an ultrasonic welding systemincludes a movable ultrasonic welding stack to move and to applyvibration energy to at least one workpiece responsive to control inputs.The system also includes a motion control system to control a motion ofthe ultrasonic welding stack. The system further includes a controllerto provide control inputs to the motion control system associated withthe ultrasonic welding stack to control movement of the ultrasonicwelding stack and to control an output of the ultrasonic welding stackto the at least one workpiece. The system further includes a sensor tosense the force on the parts being joined and to output the force to thecontroller. The controller causes the ultrasonic welding stack,following the initiation of the welding, to automatically change thespeed of motion of the ultrasonic welding stack from a speed changeinitiating condition, based on a predetermined value of the rate ofchange of force, until a speed change terminating condition, based on adifferent value of the rate of change of force.

According to another exemplary embodiment D2, the initiating conditionof embodiment D1 is an increase in the rate of change of force above apredetermined level. The speed is reduced to a fraction of theprogrammed speed or to zero.

According to yet another exemplary embodiment D3, the initiatingcondition of embodiment D1 is a decrease in the rate of change of forcebelow a predetermined level. The speed is increased by a factor largerthan 1 relative to the programmed speed.

According to yet another exemplary embodiment D4, the terminatingcondition of embodiment D1 is a decrease of the rate of change of forcebelow a predetermined level.

According to yet another exemplary embodiment D5, the terminatingcondition of embodiment D1 is an increase in the rate of change of forceabove a predetermined level.

According to yet another exemplary embodiment D6, the terminatingcondition of embodiment D1 is a return of the rate of change of force tothe level of the initiating condition.

Referring to FIG. 16, another alternative embodiment includes dynamicspeed adjustment to limit forces during a weld cycle. During the weldand hold (solidification) phases, it is beneficial in some cases toprevent the force between the parts being joined from becomingexcessively large or excessively small. When directly controlling thespeed of motion of the ultrasonic stack, the force can be influenced byautomatically changing the speed based on an algorithm and input from aforce sensor. If the force increases to a predetermined upper level, thepress speed is reduced. Conversely, if the force decreases to apredetermined lower level, the press speed is increased.

The amount of speed change can be a predetermined fraction ormultiple-factor of the current speed. By way of example, the amount ofspeed change can include a reduction in speed of 100% relative to thecurrent speed or an increase in speed in excess of 100% of the currentspeed.

According to the example illustrated in FIG. 16, graphs of weld speedand force are plotted versus time and illustrate a weld without (a) and(b) with dynamic speed adjustment. Referring to the weld without thedynamic speed adjustment, the speed during the weld is constant atS_(w), which results in a continuous increase in the force. When dynamicspeed adjustment is employed, the speed is automatically reduced to aprescribed level 0.5 S_(w) when the force reaches the predeterminedupper value F_(u), which is represented at time t₁. Subsequently, as theweld continues at the reduced speed, the force decreases and eventuallyreaches the predetermined lower value F₁, which is represented at timet₂. At this point in time, the speed automatically reverts to theprogrammed weld speed S_(w).

Referring to the weld with the dynamic speed adjustment, the approach ofchanging the speed once the force reaches predetermined levels mayresult in an overshoot in the force above and/or below the predeterminedlevels because speed changes are not instantaneous. If tighter controlor avoidance of the overshoot is desired, the automatic speed adjustmentalgorithm (utilizing other input parameters in addition to force) can beconfigured to forecast the timing and magnitude of the speed changesneeded to avoid overshoot and, thus, maintain the force within thepredetermined limits. Exemplary additional input parameters include, butare not limited to, the rate of change of force and/or the rate ofchange of ultrasound power.

According to an exemplary embodiment E1, an ultrasonic welding systemincludes a movable ultrasonic welding stack to move and to applyvibration energy to at least one workpiece responsive to control inputs.The system also includes a motion control system to control a motion ofthe ultrasonic welding stack. The system further includes a controllerto provide control inputs to the motion control system associated withthe ultrasonic welding stack to control movement of the ultrasonicwelding stack and to control an output of the ultrasonic welding stackto the at least one workpiece. The system further includes one or moresensors to sense at least one control variable and to output at leastone signal corresponding to the at least one control variable to thecontroller. The controller causes the ultrasonic welding stack,following the initiation of the welding, to automatically change thespeed of motion of the ultrasonic stack from a speed change initiatingcondition, based on a predetermined value of the control variable, untila speed change terminating condition, based on a different value of thecontrol variable.

According to another exemplary embodiment E2, one or more sensors ofembodiment E1 sense a force output by the linear actuator movableelement. The initiating condition is an increase in the force above apredetermined level and the terminating condition is a decrease in theforce below a predetermined level.

According to yet another exemplary embodiment E3, one or more sensors ofembodiment E1 sense a force output by the linear actuator movableelement. The initiating condition is a decrease in the force below apredetermined level and the terminating condition is an increase in theforce above a predetermined level.

According to yet another exemplary embodiment E4, one or more sensors ofembodiment E1 sense a force output by the linear actuator movableelement and a power input to the ultrasonic stack. The initiatingcondition is a first function based on input from one or more of thesensors, and the terminating condition is a second function based oninput from one or more of the sensors.

According to yet another exemplary embodiment E5, one or more sensors ofembodiment E1 sense a force output by the linear actuator movableelement, a power input to the ultrasonic welding stack, or both theforce output and the power input. The initiating condition is a firstfunction and the terminating condition is a second function. The firstfunction is based on input from at least one sensor, the rate of changeof output from at least one sensor, or a combination thereof. The secondfunction, which is distinct from the first function, is based on inputfrom at least one sensor, the rate of change of output from at least onesensor, or a combination thereof.

Referring to FIG. 17, yet another alternative embodiment includes speedramp-up at the start of a weld. In a typical ultrasonic weld, theamplitude of vibration is gradually increased from zero at the start ofthe weld to a predetermined weld amplitude, with the rate of increasebeing generally constant. During the amplitude ramp-up interval, theamount of ultrasonic energy available to be transferred to the partsbeing welded is limited. If the press is programmed to travel at aconstant speed while the amplitude is ramping up, the plastic may notmelt sufficiently fast (which can cause the force between the parts tobecome excessively high). The motion delay techniques described aboveare effective ways to prevent force buildup.

However, an additional or alternative technique, which does not requirea force sensor, is to gradually increase the speed from a low initialvalue (including zero) to the prescribed weld speed. As illustrated inFIG. 17, one example is directed to ramping up the speed in directproportion to the actual ultrasound amplitude. According to theillustrated graphs of ultrasound amplitude and speed plotted versustime, the ultrasound amplitude is zero and the speed is at a low levelSi when the ultrasonic vibrations are initiated at the start of the weld(at time “0”).

As the ultrasound amplitude increases with time, the speed increasesproportionally until the ultrasound amplitude reaches the weld amplitudeA_(w) and the speed simultaneously reaches a weld speed S_(w) (at timet₁). Another variation of this feature is to have the speed ramp-up be anon-linear function of the amplitude (e.g., a polynomial function). Thespeed ramp-up function may also be a linear or a non-linear function ofanother sensed parameter, such as (but not limited to) ultrasound power.Although the case of a constant weld speed is depicted in FIG. 17 (asindicated after time t₁), this feature can be applied to welds where thespeed is variable following the ramp-up period or where the press force(instead of speed) is controlled.

Similarly, the present concepts are not limited to ultrasonic welding,but may advantageously be incorporated into other welding processes andwelding equipment utilizing a servo motor or actuator to driveworkpieces such as, but not limited to, friction welding or diffusionwelding.

According to an exemplary embodiment F1, an ultrasonic welding systemincludes a movable ultrasonic welding stack to move and to applyvibration energy to at least one workpiece responsive to control inputs.The system also includes a motion control system to control a motion ofthe ultrasonic welding stack. The system further includes a controllerto provide control inputs to the motion control system associated withthe ultrasonic welding stack to control movement of the ultrasonicwelding stack and to control an output of the ultrasonic welding stackto the at least one workpiece. The system further includes at least onesensor to sense at least one control variable and to output at least onesignal corresponding to the at least once control variable to thecontroller. The controller causes the ultrasonic welding stack, upon theinitiation of welding, to automatically change the speed of motion ofthe ultrasonic stack as a function of the control variable until thecontrol variable reaches a predetermined value.

According to another exemplary embodiment F2, the sensor of embodimentF1 senses an ultrasound amplitude of the ultrasonic welding stack. Thespeed is linearly proportional to the ultrasound amplitude.

According to yet another exemplary embodiment F3, the sensor ofembodiment F1 senses a power input to the ultrasonic welding stack. Thespeed is linearly proportional to the power input.

According to yet another exemplary embodiment F4, the sensor ofembodiment F1 senses an ultrasound amplitude of the ultrasonic weldingstack. The speed is a non-linear function of the ultrasound amplitude.

According to yet another exemplary embodiment F5, the sensor ofembodiment F1 senses a power input to the ultrasonic welding stack. Thespeed is a non-linear function of the power input.

According to yet another exemplary embodiment F6, one or more sensors ofembodiment F1 sense one or more of a frequency of the ultrasonic stackand a phase of a transducer of the ultrasonic welding stack. The speedis a linear or non-linear function of frequency and/or phase.

According to yet another exemplary embodiment F7, one or more sensors ofembodiment F1 sense one or more of an ultrasound amplitude of theultrasonic welding stack, a power input to the ultrasonic welding stack,a frequency of the ultrasonic welding stack, and a phase of a transducerof the ultrasonic welding stack. The speed is a linear or non-linearfunction of any combination including one or more of the amplitude,power, frequency, phase, rate of change of amplitude, rate of change ofpower, rate of change of frequency, and/or rate of change of phase.

While particular implementations and applications of the presentdisclosure have been illustrated and described, it is to be understoodthat this disclosure is not limited to the precise construction andcompositions disclosed herein and that various modifications, changes,and variations can be apparent from the foregoing descriptions withoutdeparting from the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. An ultrasonic welding system comprising: amovable ultrasonic welding stack to move and to apply vibrational energyto at least one workpiece responsive to control inputs; an electricallypowered linear actuator including a servo motor and a movable elementcoupled to the ultrasonic welding stack, the electrically powered linearactuator causing, responsive to control inputs, the movable element andthe ultrasonic welding stack to move with a controlled force, speed, orforce and speed; a controller to provide control inputs to theelectrically powered linear actuator to control an output of theelectrically powered linear actuator; and at least two sensors to sensecontrol variables and to output signal outputs corresponding to thecontrol variables to the controller; wherein the controller causes theultrasonic welding stack to apply a predetermined positive initial forceto the at least one workpiece prior to initiation of welding and toinitiate subsequent movement of the ultrasonic welding stack, followinginitiation of welding, only after the signal outputs from the at leasttwo sensors indicate that a combination of control variables satisfies apredetermined condition.
 2. The ultrasonic welding system of claim 1,wherein the at least two sensors sense a force output by the movableelement and an amplitude of the ultrasonic welding stack, thepredetermined condition consisting of both a specified force and aspecified amplitude.
 3. The ultrasonic welding system of claim 1,wherein the at least two sensors sense a force output by the movableelement and a power input to a transducer of the ultrasonic weldingstack, the predetermined condition consisting of both a specified forceand a specified power.
 4. The ultrasonic welding system of claim 1,wherein the at least two sensors sense a force output by the movableelement and a power input to a transducer of the ultrasonic weldingstack, the predetermined condition consisting of both a specified forceand a specified cumulative power.
 5. The ultrasonic welding system ofclaim 1, wherein one of the at least two sensors senses a force outputby the movable element and another one of the at least two sensorstracks elapsed time following the initiation of the welding, thepredetermined condition consisting of both a specified force and aspecified elapsed time.
 6. The ultrasonic welding system of claim 1,wherein the at least two sensors sense an output torque of the servomotor and an amplitude of the ultrasonic welding stack, thepredetermined condition consisting of both a specified output torque anda specified amplitude.
 7. The ultrasonic welding system of claim 1,wherein the at least two sensors sense an output torque of the servomotor and a power input to a transducer of the ultrasonic welding stack,the predetermined condition consisting of both a specified output torqueand a specified power.
 8. The ultrasonic welding system of claim 1,wherein the at least two sensors sense an output torque of the servomotor and a power input to a transducer of the ultrasonic welding stack,the predetermined condition consisting of both a specified output torqueand a specified cumulative power.
 9. The ultrasonic welding system ofclaim 1, wherein one of the at least two sensors senses an output torqueof the servo motor and another one of the at least two sensors trackselapsed time following the initiation of the welding, the predeterminedcondition consisting of both a specified output torque and a specifiedelapsed time.
 10. The ultrasonic welding system of claim 1, wherein theat least two sensors sense one or more parameters selected from a groupconsisting of a force output by the movable element, an output torque ofthe servo motor, an amplitude of the ultrasonic welding stack, a powerinput to a transducer of the ultrasonic welding stack, a cumulativepower input to the transducer, a frequency of the ultrasonic weldingstack, a phase of the transducer, and an elapsed time from theinitiation of the welding; wherein the predetermined condition consistsof simultaneously satisfying two criteria, each of the two criteriabeing associated with a distinct parameter of the one or moreparameters; wherein each of the two criteria is associated with adistinct parameter of the one or more parameters, each of the twocriteria being selected from a group consisting of a specified force, aspecified output torque, a specified amplitude, a specified power, aspecified cumulative power, a specified frequency, a specified phase,and a specified elapsed time.
 11. The ultrasonic welding system of claim1, wherein the at least two sensors sense at least two parametersselected from a group consisting of: (a) a rate of change of a powerinput to a transducer of the ultrasonic welding stack, (b) a rate ofchange of a frequency of the ultrasonic welding stack, (c) a rate ofchange of a phase of the transducer, (d) a rate of change of a force ofthe movable element, and (e) a rate of change of an output torque of theservo motor; wherein the predetermined condition is (i) a specified rateof change of power or a specified rate of change of cumulative power ifone of the at least two parameters is the rate of change of the powerinput to the transducer, (ii) a specified rate of change of frequency ifone of the at least two parameters is the rate of change of thefrequency of the ultrasonic welding stack, (iii) a specified rate ofchange of phase if one of the at least two parameters is the rate ofchange of the phase of the transducer, (iv) a specified rate of changeof force if one of the at least two parameters is the rate of change ofthe force of the movable element, and (v) a specified rate of change ofoutput torque if one of the at least two parameters is the rate ofchange of the output torque of the servo motor.
 12. A method for awelding operation, the method comprising: initiating a welding operationby moving, in response to control inputs, an ultrasonic welding stack toapply vibrational energy to a workpiece; causing, responsive to controlinputs, a movable element of an electrically powered linear actuator tomove with a controlled force, speed, or force and speed; providingcontrol inputs, via a controller, to the electrically powered linearactuator for controlling an output of the electrically powered linearactuator; sensing control variables, via at least two sensors, andoutputting signal outputs corresponding to the control variables to thecontroller; causing, via the controller, the ultrasonic welding stack toapply a predetermined initial force to the at least one workpiece priorto initiation of welding; initiating subsequent movement of theultrasonic welding stack, following initiation of welding, only afterthe signal outputs from the at least two sensors indicate that acombination of control variables satisfies a predetermined condition.13. The method of claim 12, further comprising sensing, via the at leasttwo sensors, a force output by the movable element and an amplitude ofthe ultrasonic welding stack, the predetermined condition consisting ofboth a specified force and a specified amplitude.
 14. The method ofclaim 12, further comprising sensing, via the at least two sensors, aforce output by the movable element and a power input to a transducer ofthe ultrasonic welding stack, the predetermined condition consisting ofboth a specified force and a specified power.
 15. The method of claim12, further comprising sensing, via the at least two sensors, a forceoutput by the movable element and a power input to a transducer of theultrasonic welding stack, the predetermined condition consisting of botha specified force and a specified cumulative power.
 16. The method ofclaim 12, further comprising sensing, via one of the at least twosensors, a force output by the movable element and tracking, via anotherone of the at least two sensors, elapsed time following the initiationof the welding, the predetermined condition consisting of both aspecified force and a specified elapsed time.
 17. The method of claim12, further comprising sensing, via the at least two sensors, an outputtorque of the servo motor and an amplitude of the ultrasonic weldingstack, the predetermined condition consisting of both a specified outputtorque and a specified amplitude.
 18. The method of claim 12, furthercomprising sensing, via the at least two sensors, an output torque ofthe servo motor and a power input to a transducer of the ultrasonicwelding stack, the predetermined condition consisting of both aspecified output torque and a specified power.
 19. The method of claim12, further comprising sensing, via the at least two sensors, an outputtorque of the servo motor and a power input to a transducer of theultrasonic welding stack, the predetermined condition consisting of botha specified output torque and a specified cumulative power.
 20. Themethod of claim 12, further comprising sensing, via the at least twosensors, at least two parameters selected from a group consisting of:(a) a rate of change of a power input to a transducer of the ultrasonicwelding stack, (b) a rate of change of a frequency of the ultrasonicwelding stack, (c) a rate of change of a phase of the transducer, (d) arate of change of a force of the movable element, and (e) a rate ofchange of an output torque of the servo motor; wherein the predeterminedcondition is (i) a specified rate of change of power or a specified rateof change of cumulative power if one of the at least two parameters isthe rate of change of the power input to the transducer, (ii) aspecified rate of change of frequency if one of the at least twoparameters is the rate of change of the frequency of the ultrasonicwelding stack, (iii) a specified rate of change of phase if one of theat least two parameters is the rate of change of the phase of thetransducer, (iv) a specified rate of change of force if one of the atleast two parameters is the rate of change of the force of the movableelement, and (v) a specified rate of change of output torque if one ofthe at least two parameters is the rate of change of the output torqueof the servo motor.